1. Field of the Invention
The present invention relates generally to a photovoltaic device, and more particularly, to a photovoltaic device having a heterojunction formed by combining a crystalline semiconductor and an amorphous semiconductor.
2. BACKGROUND INFORMATION
In recent years, solar cells using polycrystalline silicon have been extensively studied as photovoltaic devices. Of the solar cells, a solar cell having a heterojunction formed by combining amorphous silicon and polycrystalline silicon has, received attention because it is low in cost and high in conversion efficiency.
In the above described solar cell having the heterojunction, a large number of interfacial states may often be formed in the vicinity of the junction interface of amorphous silicon and crystalline silicon. Therefore, even if light is incident on the junction to generate a large number of carriers, the carriers disappear at recombination centers due to the above described interfacial states, resulting in a reduced conversion efficiency.
In order to decrease the number of interfacial states in the vicinity of the heterojunction interface to improve junction characteristics, a photovoltaic device having an intrinsic amorphous semiconductor layer disposed in the heterojunction interface has been proposed, in Japanese Patent Laying-Open Nos. 4-130671 (1992) and 4-199750 (1992).
However, the structure having the intrinsic amorphous semiconductor layer in the heterojunction interface is not different from the structure without the intrinsic amorphous layer, in that semiconductors, having different forbidden band widths in the vicinity of the junction interface are coupled to each other. Consequently, energy barriers against photo-generated carriers still exist, so that the collection efficiency of the carriers is still insufficient.
Furthermore, the amorphous semiconductor layer is relatively high in electrical resistance, thereby making it difficult for the carriers to move smoothly moved in the amorphous semiconductor layer. As a result, the loss at the interface of the crystalline semiconductor and the amorphous semiconductor cannot be ignored.